While everyone has been busy wondering when Facebook was going to IPO, most were looking past the first question: how is Facebook going to IPO? But not TechCrunch alum Evelyn Rusli and Andrew Ross Sorkin. Tonight the pair are reporting that Goldman Sachs has just led a major new investment in the social network. An investment that values it at a nice round $50 billion. And the likely reason is so Goldman can take Facebook public.

More specifically, Goldman has invested $450 million in Facebook while Russian firm (and current large stakeholder) Digital Sky Technologies threw in another $50 million for a total of $500 million in this round. But the round is more complicated than that as apparently Goldman will be able to unload some of its stake to DST, according to the report.

So is the IPO a done deal? Not exactly. While Facebook itself hasn’t given a timetable for the milestone, many were expecting it to happen in 2011. But more recent indications suggest it will more likely occur in 2012. Facebook, obviously, wants it to happen as late as possible so they don’t have shareholder to answer to. But that’s the thing, with all the investment they’ve taken, and the red-hot activity on the secondary markets, they already do have a lot of shareholders â€” they just don’t technically have to answer to them.

Yet.

The SEC is said to be looking into whether or not that Facebook is improperly skirting around the rules for going public.

Either way, Goldman looks to make a a good chunk of change before the IPO as well. From the report:

In a rare move, Goldman is planning to create a â€œspecial purpose vehicleâ€ to allow its high-net worth clients to invest in Facebook, these people said. While the S.E.C. requires companies with more than 499 investors to disclose their financial results to the public, Goldmanâ€™s proposed special purpose vehicle may be able get around such a rule because it would be managed by Goldman and considered just one investor, even though it could conceivably be pooling investments from thousands of clients.

If that doesn’t piss the SEC off too much, that is huge for the firm. That might not be so easy â€” with the special purpose vehicle option, Goldman is said to be trying to raise up to $1.5 billion for Facebook at that $50 billion valuation.

Remember that it was barely a month ago when we reported on Accel Partners selling off a big portion of their Facebook stock at a $35 billion valuation. Of course, they made something like a 247x return on that sale, so it’s hard to argue with it. A few weeks later, we noted that Facebook had hit the $50 billion mark in valuation on the secondary markets. With this investment, that valuation has just been validated.

As if The Social Network needed any more buzz leading up to its DVD release next week…